The present invention relates to substrate processing equipment and more particularly to plasma processing equipment for performing plasma processing steps, such as deposition, clean, and/or etch processes on a process substrate.
Plasma processing methods are used in the fabrication of semiconductor devices, such as integrated circuits, and other types of substrates, such as micro-electro-mechanical (xe2x80x9cMEMsxe2x80x9d) substrates to achieve a variety of results. Plasma methods include the formation of a layer using plasma-enhanced chemical vapor deposition and etching techniques, such as reactive ion etching. Additionally, a plasma might be used to clean a processing chamber or to prepare a surface of a substrate for a subsequent process step, such as a plasma wafer surface clean or activation prior to formation of a layer on the surface.
The wide application of plasma processing has resulted in a wide variety of plasma processing systems and apparatus. One type of plasma processing chamber places the wafer on an electrode of the plasma circuit, opposite another planar electrode, and capacitively couples high-frequency electrical power to the two electrodes to form a plasma between them. Such a plasma reactor has advantages where it is desirable to form the plasma in the presence of the substrate, such as when the physical movement of plasma species to and from the substrate is desired. However, some devices or materials might not be compatible with this type of plasma formation, particularly the bombardment by plasma species, including high-energy photons, and associated heating of the substrate.
Another approach to plasma processing generates plasma in a remote location, and couples the plasma to a processing chamber. Various types of plasma generators have been developed, including magnetron sources coupled to a cavity, inductively coupled toroidal sources, microwave irradiation directed at a plasma precursor, electron-cyclotron resonance generators, and others. Remote plasma techniques offer a number of advantages for certain types of processes, such as cleaning deposition chambers, but generally the atomic species that eventually reach the chamber are of relatively low density, due to recombination of the reactive plasma species with each other or with components of the processing system, such as the chamber walls or delivery conduit.
Inductively coupled plasma systems have been developed that can generate a high-density plasma in one portion of the processing chamber (e.g. above the wafer), yet shield the wafer from the more deleterious effects of the plasma generation process by using the plasma itself as a buffer between the wafer and the plasma generation region and typically relies on diffusion of plasma to provide a uniform ion density across the wafer surface. In one system, a dielectric dome, or chamber top, has a conductive coil wound around the dome. High-frequency electric energy provided to the coil couples to a plasma precursor gas in the chamber and converts the precursor to plasma. In some systems, a second power supply couples an alternating field to the wafer or wafer support structure, and allows a directional component to and from the wafer to be added to the plasma generated by the coils. Such systems are used for both deposition and etch processes to achieve desirable results, generally providing both high rates and good uniformity across a wafer.
However, the fields generated by the coil through the dome have an electric field component normal to the surface of the dome that causes plasma species to be directed to and from the inner surface of the dome. This field component acting on the plasma can cause physical erosion (xe2x80x9csputteringxe2x80x9d) of the inside of the dome, as well as affect the power coupling to the plasma, thus causing a non-uniform plasma density. In some instances the plasma might contain species that react with the material of the dome, further eroding the dome and potentially creating particles than can fall from the dome onto the wafer, creating defects. Reaction of the dome material with the plasma often arises in an etch process when the material being etched is similar to the material of the dome, e.g. silica-based glass. If erosion of the inner surface of the dome continues to a point where particulate contamination or strength of the dome is an issue, the dome might have to be replaced, affecting through-put of the plasma system, and potentially disrupting the product flow through an entire fabrication line.
Thus, it is desirable to provide a plasma processing system that avoids the surface erosion problem of conventional systems while creating a high-density, uniform plasma.
The present invention provides a plasma processing apparatus applicable to deposition, etch, and/or cleaning processes. Such processes may be applied to a substrate, such as a silicon wafer, composite wafer, glass panel, or other materials. In some instances, the plasma generated by the apparatus might be used for chamber cleaning purposes, in the absence of a substrate.
In one embodiment of the invention, a toroidal plasma source within a plasma processing chamber forms a poloidal plasma current. The toroidal plasma source includes a transformer having a primary circuit including a coil or coupling structure, a toroidal core, and a secondary circuit, when in operation, of plasma within the chamber. In a particular embodiment, a ferrite core contained within the processing chamber is approximately circular, resulting in a plasma having theta symmetry. In an alternate embodiment, the primary coil forms a toroid, with an xe2x80x9cairxe2x80x9d core. When processing a substrate, the process surface of the substrate can be oriented in a plane essentially parallel to the plane of the toroidal plasma source, taking advantage of the theta symmetry for improved process uniformity. The electric field that sustains the plasma is substantially parallel to all surfaces of the chamber, creating the poloidal current, thus reducing or eliminating reactive ion etching or sputtering of the surfaces of the chamber. The poloidal plasma current minimizes sputtering erosion of the chamber components surrounding the plasma source, as well as the surface of the substrate.
In a further embodiment, an AC or DC bias field can be applied to the substrate, relative to the plasma, to further control the interaction of the plasma with the surface of the substrate. In another embodiment, a shaped surface between the toroidal core and the process surface of the substrate modifies the plasma density in a selected fashion, typically along a radius of the substrate surface. For example, the cross section of the plasma conduit path around the toroidal plasma source could be narrowed near the outer perimeter of the substrate to increase plasma density in this region.
These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures.